The cyclic tetradecapeptide somatostatin-14 (SRIF) was originally isolated from the hypothalamus and characterized as a physiological inhibitor of growth hormone release from the anterior pituitary. SRIF is localized throughout the central nervous system, where it acts as a neurotransmitter and has been shown to both positively and negatively regulate neuronal firing, to affect the release of other neurotransmitters, and to modulate motor activity and cognitive processes.
Somatostatin and many analogs of somatostatin exhibit activity in respect to the inhibition of growth hormone (GH) secretion from cultured, dispersed rat anterior pituitary cells in vitro; they also inhibit GH, insulin and glucagon secretion in vivo in the rat and in other mammals. One such analog is [D-Trp8]-SRIF which has the amino acid sequence: (cyclo 3-14)H-Ala-Gly-Cys-Lys-Asn-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-Ser-Cys-OH, which is disclosed in U.S. Pat. No. 4,372,884 (Feb. 8, 1983). Somatostatin has also been found to inhibit the secretion of gastrin and secretin by acting directly upon the secretory elements of the stomach and pancreas, respectively, and somatostatin is being sold commercially in Europe for the treatment of ulcer patients. SRIF is also known to inhibit the growth of certain tumors.
SRIF induces its biological effects by interacting with a family of membrane-bound structurally similar receptors. Five SRIF receptors have been cloned and are referred to as SSTR1–5. All five receptors bind SRIF and SRIF-28 (an N-terminally extended version) with high affinity; however, studies have now shown that different receptor subtypes mediate distinct functions of SRIF in the body.
A cyclic SRIF analog, variously termed SMS-201-995 and Octreotide, i.e. D-Phe-c[Cys-Phe-D-Trp-Lys-Thr-Cys]-Thr-ol is being used clinically to inhibit certain tumor growth; analogs complexed with 111In or the like are also used as diagnostic agents to detect SRIF receptors expressed in cancers. Two similar octapeptide analogs having 6-membered rings, i.e. Lanreotide and Vapreotide, have also been developed, see Smith-Jones et al., Endocrinology, 140, 5136–5148 (1999). A number of versions of these somatostatin analogs have been developed for use in radioimaging or as radiopharmaceuticals in radionuclide therapy; for radioimaging, for example, labeling with 125I can be used. Proteins have been previously radiolabeled through the use of chelating agents, and there are various examples of complexing somatostatin analogs with 99Tc, 90Y or 111In. A variety of complexing agents have been used including DTPA (Virgolini, et al., European Journal of Nuclear Medicine, 23:1388–1399, October 1996); (Stabin, et al., J. Nuc. Med., 38:1919–1922, December 1997); (Vallabhajosula, et al., J. Nuc. Med., 37:1016–1022, June 1996); DOTA (De Jong, et al., Int. J. Cancer, 75:406–411, 1998); (Froidevaux, et al., Peptide Science-Present and Future, 670–673, 1999); HYNIC (Decristoforo, et al. Eur. J. Nuc. Med., 26:869–876); (Krois, et al., Liebigs Ann., 1463–1469, 1996); and P2S2-COOH (Karra, et al., Bioconjugate Chem., 10:254–260, 1999. Another such complexing/conjugating agent sometimes used is succinimidyl 6-hydrazinium nicotinate hydrochloride (SHNH). A wide variety of such complexing/conjugating agents are disclosed and discussed in U.S. Pat. No. 5,972,308 (Oct. 26, 1999).
Interventional treatment to the arterial system, such as angioplasty or bypass surgery, can be damaging to the vessel wall. Injuries to the vessel wall lead to a complex cascade of reparative responses starting with mural thrombi formation. Death of medial smooth muscle cells (SMC) from vessel wall injury also initiates the release of growth factors, such as basic fibroblast growth factor (bFGF) and platelet-derived growth factor from platelets, macrophages, and endothelial cells. These factors subsequently stimulate the proliferation and migration of medial SMC into the intima. Prolifering SMC synthesize and secrete a wide variety of mitogenic growth factors to promote further SMC proliferation and elaboration of extracellular matrix. This reparative process terminates when the damaged endothelium has been restored. However, if cell proliferation and matrix deposition continue, they lead to a pathological condition known as intimal hyperplasia (IH).
Clinically, IH causes renarrowing, or restenosis, of treated arteries in 30–50% of coronary angioplasties within six months and in ˜20% of bypass procedures within two years after treatment. Apart from intravascular stents and anticoagulation, which appear effective in limiting restenosis in the short term, no other interventions have been successful in halting the development of IH; however, there is now evidence that SRIF may have an effect upon IH.
Recent evidence indicates that a somatostatin (SRIF) analog, angiopeptin (BIM-23014), is effective in inhibiting IH after arterial injury in animal models. Angiopeptin inhibits the release of insulin-like growth factor-1 and bFGF from endothelial cells, thus preventing SMC proliferation and migration. Clinical trials using angiopeptin to inhibit IH-causing restenosis, however, have been inconclusive. Angiopeptin is selective for SSTR2, SSTR3 and SSTR5, and IH appears to be mediated by SSTR1, which may explain the inconclusive results.
Octreotide, angiopeptin and other clinically used SRIF analogs interact significantly with three of the receptor subtypes, i.e. SSTR2, SSTR3 and SSTR5. SSTR2 and SSTR5 have recently been reported to mediate antiproliferative effects of SRIF on tumor cell growth; therefore, they may mediate the clinical effects of Octreotide in humans. A recent comprehensive review of SRIF and its receptors is found in Patel, Y. C. “Somatostatin and its receptor family”, Front. Neuroendocrinol, 1999, 20, 157–198. Pending U.S. patent application Ser. No. 09/607,546, filed Jun. 29, 2000, discloses SSTR3-selective synthetic analogs of SRIF. U.S. Pat. No. 5,750,499 (May 12, 1998) discloses SRIF analogs which are selective for SSTR1, and since that discovery, efforts have been made to discover analogs with even greater selectivity and/or greater binding strength.
Nonpeptide SRIF agonists have been identified using combinatorial chemistry which exhibit selectivity for each of SSTR1 to SSTR5, Rohrer, S. P. et al., Science,282, 737–740, Oct. 23, 1998. However, improved peptide ligands continue to be sought because only peptide ligands can be satisfactorily derivatized to incorporate complexing agents for radionuclides. Additionally, peptides generally exhibit fewer undesirable side effects, such as toxicity or cross reactivity with unrelated receptors.